Week 7: Terrain Analysis

Yi Qiang
Feb. 24, 2017

Terrain Analysis

  • The Earth Surface is not flat

Terrain Analysis

  • In addition to X, Y, there is Z (height)
  • Also called 3D Analysis

Why analyze terrain?

  • Different types of terrain - different patterns of X, Y, and Z
  • Formed by different processes - geological, hydrological, and volcanic
  • Human activities (anthropocene)

Why analyze terrain?

Spatial metrics, such as distance, area and volumne

Why analyze terrain?

Surface adjusted distance

Why analyze terrain?

hydrological modeling

Why analyze terrain?

Landslide modeling

Why analyze terrain?

Suitability modeling

Modeling Surface in GIS

Two Most Common Terrain Data Models:

  • Digital Elevation Data (DEM) (Raster)
  • Triangular Irregular Network (TIN) (Vector)

Digital Elevation Model (DEM)

  • DEM is basically a normal raster of elevation -it's called DEM because it is specific for a topographic surface
  • A DEM is the simplest form to represent a topographic surface
  • One of the most widely used data layers in many GIS applications
  • The critical parameter is the resolution (or grid cell size)
  • 1m resolution DEM for HI, 30m resolution for the US and 1km resolution for the globe are freely available

Digital Elevation Model (DEM)

Essentially a raster of elevation (2D matrix)

Digital Elevation Model (DEM)

DEM in 3D

Triangular Irregular Model

A vector model for 3D terrain

Triangular Irregular Model

Delaunay Triangulation between sampling points

Triangular Irregular Model

A facet of TIN in 3D

Triangular Irregular Model

Resolution of TIN - number of triangles

Basic Terrain Analysis

  • Surface-adjusted metrics (distance and area)
  • Slope (Landslide susceptibility)
  • Aspect (Solar insolation, vegetation)
  • Profile (elevation change along a line)
  • Viewshed (visibility)
  • Hillshade (visualization)
  • Area solar radiation
  • Flow path, watershed delination, flow accumulation (hydrology)
  • Indices (e.g., terrain ruggedness)

Distance calculation in DEM (Pixel-to-pixel approach)

  • Convert a vector line to a raster line
  • The sum of actual run between every pair of adjacent pixels

Distance calculation in DEM (Sampling approach)

  • Define sampling points along a transect
  • The sum of distance between each pair of points

Distance calculation in TIN

  • Define a number of sampling points along a transect
  • The elevation of a point is estimated from the triangle it is located in or the surrounding triangles
  • The sum of distance between each pair of sampling points

No guidelines of distance calculation

  • ~ 10 different measurement approaches
  • Caculation accuracy = f(DEM/TIN resolution, landscape, interpolation method, sampling size)
  • Trade-offs between accuracy and data size, accuracy and computation time.

Area caculation

Surface adjusted area is the total area of all the triangular facets

Slope Calculation

  • Angle of the slope (degree)
  • Ratio of the slope (percentage)

Calculate slope in DEM

  • In DEM, maximum rate of change in value from that cell to its neighbors.

Slope calculation in ArcGIS

Geometric combination of vertical (x direction) descent and horizontal (y direction) descent

Calculate slope in DEM

Elevation to Slope

Aspect

  • The direction of the slope


Profile

Most GIS packages provide tools for examining the profile of a surface along a selected straight line or series of straight line segments (polyline).

Profile

Profile along multiple straight transects

Profile

  • Profile of a path/route

Viewshed

  • Areas visible to a set of observer features.
  • Viewshed tool in ArcGIS

Viewshed

  • Listening Posts/Observation Posts (LP/LO)
  • Weapons placement
  • Antenna placement for radar, radios, or cell phones
  • Special surveillance equipment placement

Hillshade

  • Hypothetical illumination of a surface by determining illumination values for each cell in a raster
  • Enhance the visualization of a surface for analysis or graphical display
  • Parameters: altitude and azimuth

Altitude (Hillshade)

  • The altitude is the angle of the illumination source (e.g. the sun) above the horizon.
  • The units are in degrees, from 0 (on the horizon) to 90 (overhead). The default is 45 degrees.

Hillshade (Hillshade)

  • Azimuth is the angular direction of the sun, measured from north in clockwise degrees from 0 to 360.
  • An azimuth of 90 degrees is east. The default azimuth is 315 degrees (NW).
  • Why default azimuth is 315 (NW), given most people are living in the North Hemisphere where the Sun azimuth is South?

Hillshade

Sink or hill?

Hillshade

Sink or hill?

Hillshade

Original DEM - hill

Hillshade

  • Combining viewshed with elevation

Solar Radiation Analysis

Cumulative solar radiation in a day (Area Solar Radiation tool in ArcGIS)

Viewshed

Solar Radiation: Summer Solstice

Viewshed

Solar Radiation: Winter Solstice

Anouncement

Change of topics: Topic 5: Spatial Interpolation -> Hazard risk and vulnerability assessment Topic 6: Advanced spatial modeling -> Web mapping Next class -> Project discussion

Lab 4 Assignment (Terrain analysis)